Lubricating oil composition for sliding glide surface

ABSTRACT

A lubricating oil composition for a sliding guide surface, which exhibits excellent low friction properties and extreme pressure properties so as to enable high precision machining in a machine tool, is disclosed.

FIELD OF THE INVENTION

The present invention relates to a lubricating oil composition which issuitable for lubricating a sliding guide surface of a machine tool orthe like.

BACKGROUND OF THE INVENTION

In order to carry out high precision machining by means of a machinetool, it is essential for the positioning accuracy of a feed shaft ofthe machine tool to be excellent, with micron level precision beingrequired in some cases. However, lubricating oils are used becausepositioning accuracy can deteriorate due to friction resistance producedon a guide surface of a machine tool having a sliding guide surface, andit is necessary for a lubricating oil used on this guide surface toexhibit low friction.

In addition, lubricating oils used in machine tools can also be used tolubricate gears and the like in addition to guide surfaces as describedabove, and in such cases load bearing properties are also required as animportant feature.

Therefore, because smooth movement and high precision are required ofguide surfaces, a variety of friction-reducing agents are blended inlubricating oils used on guide surfaces in order to reduce friction. Forexample, JP 11-505283 discloses that attempts have been made to achievelow friction properties and good sliding properties by usingcombinations of acidic esters of phosphoric acid and phosphoric acidesters.

SUMMARY OF INVENTION

Conventional lubricating oil compositions have yet to achievesatisfactory lubricating properties for machine tools for which highprecision machining is required, and an objective of the presentinvention, which has been devised with these circumstances in mind, isto obtain a lubricating oil composition having further improvedfrictional properties and extreme pressure properties.

As a result of various investigations and research carried out with theaim of reducing friction and achieving good extreme pressure properties,as described above, it was found that in cases where a combination of aphosphonic acid ester and a fatty acid was used, a lower coefficient offriction and higher load bearing properties could be achieved than incases where either of these additives was used in isolation, and thepresent invention was completed on the basis of these findings.

The present invention provides a lubricating oil composition for asliding guide surface, which contains any of a base oil of group I, abase oil of group II, a base oil of group III or a base oil of group IVin the API (American Petroleum Institute) base oil categories, or amixture thereof, as a base oil, and which is obtained by adding, to thisbase oil, a combination of a phosphonate (phosphonic acid ester) and amiddle or higher fatty acid.

In addition, it is more preferable for the base oil to be a group IIIbase oil that is a highly refined mineral oil or a base oil that is agroup IV synthetic oil in the API base oil categories, or a mixturethereof.

The lubricating oil composition of the present invention can exhibitexcellent frictional properties and load bearing properties on a slidingguide surface of a machine tool or the like, and can be effectively usedas a lubricating oil composition for a sliding guide surface.

DETAILED DESCRIPTION OF THE INVENTION

A base oil of group I to group IV in the API base oil categories, or amixture thereof, is used in the base oil of the grease of the presentinvention. An example of a group I base oil is a paraffin-based mineraloil obtained by subjecting a lubricating oil distillate, which isobtained by subjecting crude oil to atmospheric distillation, to anappropriate combination of refining procedures, such as solventrefining, hydrorefining and dewaxing.

The viscosity index is suitably from 80 to 120, and preferably from 95to 110. The kinematic viscosity at 40° C. is preferably from 2 to680mm²/s, and more preferably from 8 to 220mm²/s. In addition, the totalsulfur content is suitably greater than 300ppm and less than 700 ppm,and preferably less than 500ppm. The total nitrogen content is suitablyless than 50ppm, and preferably less than 25ppm. Furthermore, theaniline point should be from 80 to 150° C., and preferably from 90 to120° C.

An example of a group II base oil is a paraffin-based mineral oilobtained by subjecting a lubricating oil distillate, which is obtainedby subjecting crude oil to atmospheric distillation, to an appropriatecombination of refining procedures, such as hydrocracking and dewaxing.

The viscosity of these base oils is not particularly limited, but theviscosity index is suitably from 80 to less than 120, and preferablyfrom 100 to less than 120. The kinematic viscosity at 40° C. ispreferably from 2 to 680 mm²/s, and more preferably from 8 to 220 mm²/s.

In addition, the total sulfur content is suitably no greater than300ppm, preferably no greater than 200 ppm, and more preferably nogreater than 10 ppm. The total nitrogen content is suitably less than 10ppm, and preferably less than 1 ppm. Furthermore, the aniline point issuitably from 80 to 150° C., and preferably from 100 to 135° C.

In addition, a group II base oil that has been refined using ahydrorefining process such as that used by Gulf Oil suitably has a totalsulfur content of less than 10 ppm and an aromatics content of 5% orless, and can be advantageously used in the present invention.

Examples of group III base oils include a paraffin-based mineral oilproduced by subjecting a lubricating oil distillate, which is obtainedby subjecting crude oil to atmospheric distillation, to a high degree ofhydrorefining, a base oil obtained by refining a wax, which is producedin a dewaxing process, using an isodewax process in which conversion anddewaxing are carried out, or a base oil that has been refined using thewax isomerization process used by Mobil Oil. The viscosity of thesegroup III base oils is not particularly limited, but the viscosity indexshould be from 120 to 180, and preferably from 130 to 150. The kinematicviscosity at 40° C. is preferably from 2 to 680 mm²/s, and morepreferably from 8 to 220 mm²/s. In addition, the total sulfur content issuitably 300ppm or less, and preferably 10 ppm or less. The totalnitrogen content is suitably 10 ppm or less, and preferably 1 ppm orless. Furthermore, the aniline point is suitably from 80 to 150° C., andpreferably from 110 to 135° C.

In addition, as a base oil belonging to group III, a GTL (gas to liquid)base oil synthesized by the Fischer-Tropsch process, which is atechnique for converting natural gas into liquid fuel, has asignificantly lower sulfur content and aromatics content and asignificantly higher paraffin proportion than a mineral oil-based baseoil refined from crude oil, and therefore exhibits excellent oxidationstability and extremely low evaporative losses, and can beadvantageously used as the base oil in the present invention.

The viscosity properties of this GTL base oil are not particularlylimited, but the viscosity index is generally from 130 to 180, and morepreferably from 140 to 175. In addition, the kinematic viscosity at 40°C. is suitably from 2 to 680 mm²/s, and more preferably from 5 to 120mm²/s. In addition, the total sulfur content is generally less than 10ppm, and the total nitrogen content is generally less than 1 ppm. Anexample of this type of GTL base oil product is SHELL XHVI™.

Polyolefins are an example of a base oil belonging to group IV, andthese include polymers of a variety of olefins, and hydrogenatedproducts thereof. Any type of olefin can be used, but examples thereofinclude ethylene, propylene, butene and α-olefins having 5 or morecarbon atoms. When producing polyolefins, it is possible to use a singleolefin in isolation or a combination of two or more types thereof.Particularly preferred are polyolefins known as poly-α-olefins (PAO).

The viscosity of these polyolefins is not particularly limited, but thekinematic viscosity at 40° C. is preferably from 2 to 680 mm²/s, andmore preferably from 8 to 220 mm²/s.

The phosphonate mentioned above is represented by formula 1 below:

In formula 1 above, R₁ is a saturated or unsaturated alkyl group, andhas 12-22 carbon atoms, and preferably 12-18 carbon atoms. R₂ is asaturated or unsaturated alkyl group having 1-18 carbon atoms. Thesealkyl groups are often linear, but may be branched.

Examples of this type of phosphonate include dimethyldodecylphosphonate, dimethyltridecyl phosphonate, dimethyltetradecylphosphonate, dimethylpentadecyl phosphonate, dimethylhexadecylphosphonate, dimethylheptadecyl phosphonate, dimethyloctadecylphosphonate, dimethylnonadecyl phosphonate, dimethyleicosyl phosphonate,tridodecyl phosphonate, tritridecyl phosphonate, tritetradecylphosphonate, tripentadecyl phosphonate, trihexadecyl phosphonate,triheptadecyl phosphonate, trioctadecyl phosphonate (tristearylphosphite: tautomer) and trioleyl phosphonate.

This type of phosphonate is suitably used at a quantity of the order ofnot less than 0.2 mass % but less than 2 mass %, and preferably not lessthan 0.5 mass % and not more than 1.5 mass %, relative to the overallquantity of the lubricating oil composition.

The fatty acid mentioned above is represented by formula 2 below.

R₃COOH   (2)

In formula 2 above, R₃ is a saturated or unsaturated alkyl group having7-17 carbon atoms.

Examples of this type of fatty acid include caprylic acid, capric acid,lauric acid, myristic acid, palmitic acid, stearic acid, isostearicacid, oleic acid, linoleic acid and linolenic acid.

This type of fatty acid is suitably used at a quantity of the order ofnot less than 0.03 mass % but less than 1 mass %, and preferably notless than 0.1 mass % and not more than 0.7 mass %, relative to theoverall quantity of the lubricating oil composition.

Metal deactivators, anti-wear agents, and the like, can also be added tothis lubricating oil composition. Examples of metal deactivators includethiadiazole derivatives, for example2,5-bis(alkyldithio)-1,3,4-thiadiazole compounds such as2,5-bis(heptyldithio)-1,3,4-thiadiazole,2,5-bis(nonyldithio)-1,3,4-thiadiazole,2,5-bis(dodecyldithio)-1,3,4-thiadiazole and2,5-bis(octadecyldithio)-1,3,4-thiadiazole;2,5-bis(N,N-dialkyldithiocarbamyl)-1,3,4-thiadiazole compounds such as2,5-bis(N,N-diethyldithiocarbamyl)-1,3,4-thiadiazole,2,5-bis(N,N-dibutyldithiocarbamyl)-1,3,4-thiadiazole and2,5-bis(N,N-dioctyldithiocarbamyl)-1,3,4-thiadiazole; and2-N,N-dialkyldithiocarbamyl-5-mercapto-1,3,4-thiadiazole compounds suchas 2-N,N-dibutyldithiocarbamyl-5-mercapto-1,3,4-thiadiazole and2-N,N-dioctyldithiocarbamyl-5-mercapto-1,3,4-thiadiazole. In some cases,it is possible to use a benzotriazole or benzotriazole derivative, abenzimidazole or benzimidazole derivative, an imidazole or imidazolederivative, a benzothiazole or benzothiazole derivative, a benzoxazolederivative, a triazole derivative, or the like. It is possible to useone or more of these metal deactivators at a quantity of approximately0.01-0.5 mass % in the lubricating oil composition.

Examples of the anti-wear agent include diisobutyl disulfide, diisobutyltrisulfide, di-t-butyl trisulfide, dioctyl trisulfide, di-t-nonyltrisulfide, di-t-benzyl trisulfide, and other polysulfides. It is alsopossible to use a sulfurized olefin, a sulfurized oil or fat, or thelike. It is possible to use one or more of these sulfur-based anti-wearagents at a quantity of from approximately 0.1 to 3 mass % in thelubricating oil composition.

In addition, these metal deactivators and anti-wear agents can be usedin isolation or in appropriate combinations thereof, and in cases wherethese are used in combination, a low coefficient of friction can beachieved, better abrasion resistance and extreme pressure properties canbe achieved, and a sliding guide surface can be effectively lubricatedunder harsh conditions.

If necessary, antioxidants such as amine-based and phenol-basedantioxidants, corrosion inhibitors, structure stabilizers, viscositymodifiers, dispersing agents, pour point depressants, anti-foamingagents and other known additives can be blended as appropriate in thelubricating oil composition of the present invention.

The viscosity grade of the lubricating oil composition for a slidingguide surface described above should be VG22 to VG220, and preferablyVG32 to VG68, according to ISO viscosity grades.

The lubricating oil composition for a sliding guide surface of thepresent invention will now be described in specific terms throughworking examples and comparative examples, but the present invention isin no way limited to these examples.

EXAMPLES

The following materials were prepared in order to produce the workingexamples and comparative examples.

Base Oils

-   Base oil 1: GTL (gas to liquid) base oil belonging to group III    (properties: kinematic viscosity at 100° C.: 7.579 mm²/s, kinematic    viscosity at 40° C.: 43.69 mm²/s, viscosity index (VI): 141, density    at 15° C.: 0.8284) (Shell XHVI-8 manufactured by Royal Dutch Shell)-   Base oil 2: Refined mineral oil belonging to group III (properties:    kinematic viscosity at 100° C.: 7.545 mm²/s, kinematic viscosity at    40° C.: 45.50 mm²/s, viscosity index (VI): 132, density at 15° C.:    0.8453) (Yu-Base 8 manufactured by SK Innovation)-   Base oil 3: PAO (poly-α-olefin) belonging to group IV (properties:    kinematic viscosity at 100° C.: 7.741 mm²/s, kinematic viscosity at    40° C.: 46.25 mm²/s, viscosity index (VI): 136, density at 15° C.:    0.8322) (Durasyn 168 manufactured by Ineos Oligomers)-   Base oil 4: Refined mineral oil belonging to group II (properties:    kinematic viscosity at 100° C.: 5.352 mm²/s, kinematic viscosity at    40° C.: 31.10 mm²/s, viscosity index (VI): 105, density at 15° C.:    0.8627)-   Base oil 5: Refined mineral oil belonging to group II (properties:    kinematic viscosity at 100° C.: 9.490 mm²/s, kinematic viscosity at    40° C.: 73.66 mm²/s, viscosity index (VI): 106, density at 15° C.:    0.8683)-   Base oil 6: Refined mineral oil belonging to group I (properties:    kinematic viscosity at 100° C.: 4.628 mm²/s, kinematic viscosity at    40° C.: 24.32 mm²/s, viscosity index (VI): 106, density at 15° C.:    0.8625)-   Base oil 7: Refined mineral oil belonging to group I (properties:    kinematic viscosity at 100° C.: 7.446 mm²/s, kinematic viscosity at    40° C.: 51.37 mm²/s, viscosity index (VI): 106, density at 15° C.:    0.8736)

Additives

-   Additive 1-1: Dimethyloctadecyl phosphonate-   Additive 1-2: Tridodecyl phosphonate-   Additive 1-3: Tristearyl phosphite-   Additive 2-1: Caprylic acid-   Additive 2-2: Lauric acid-   Additive 2-3: Stearic acid-   Additive 2-4: Oleic acid-   Additive 3: Diethyl benzylphosphonate-   Additive 4: Behenic acid-   Additive 5: Thiadiazole-   Additive 6: Di-t-dodecyl trisulfide

Working Examples 1-16 and Comparative Examples 1-12

Lubricating oil compositions for a sliding guide surface of WorkingExamples 1-16 and Comparative Examples 1-12 were prepared using thematerials mentioned above according to the compositions shown in Tables1-3 below. The blending quantities of the components are shown as mass%.

Tests Coefficient of Friction: Pendulum Type Coefficient of FrictionTest

The coefficients of friction of the lubricating oil compositions ofWorking Examples 1-16 and Comparative Examples 1-12 were measured usinga Soda type pendulum type oiliness tester manufactured by ShinkoEngineering Co., Ltd. In this test, a test oil was applied to a wearpart that was the support point of a pendulum, the pendulum was made toswing, and the coefficient of friction was determined from theattenuation of the swing. The test was carried out at room temperature(25° C.)

Evaluation of the test was carried out according to the followingcriteria:

-   A coefficient of friction of 0.110 or less was deemed to be ∘    (pass).-   A coefficient of friction of greater than 0.110 was deemed to be ×    (fail).

Flash Point

The flash points of samples of Working Examples 1-16 and ComparativeExamples 1-12 were measured five times in accordance with JIS K2265-4using a Cleveland open cup automatic flash point measurement apparatus,and the average value was determined by rounding off to 1 digit afterthe decimal point. The thermometer used was a no. 32 thermometerspecified in JIS B7410 (COC).

Evaluation of the test was carried out according to the followingcriteria:

-   A flash point of 220° C. or higher was deemed to be ∘ (pass).-   A flash point of less than 220° C. was deemed to be × (fail).

Load Bearing Properties Test: Shell Four-Ball EP Test

Working Examples 1 and 12 and Comparative Examples 5 and 6 weresubjected to a load bearing test in accordance with ASTM D2783.

-   Conditions: Speed of rotation: 1760±40 rpm-   Duration: 10 seconds-   Temperature: room temperature-   Test items: ISL (Initial Seizure Load, units kgf) and WL (Weld Load,    units kgf).-   Test method: numerical values were determined by applying loads of    50 kgf, 63 kgf, 80 kgf, 100 kgf, 126 kgf, 160 kgf, 200 kgf, 250 kgf    and 315 kgf up to the WL.

Evaluation of the ISL was carried out according to the followingcriteria:

-   80 kgf or more was deemed to be ∘ (pass).-   Less than 80 kgf was deemed to be × (fail).

In addition, evaluation of the WL was carried out according to thefollowing criteria:

-   126 kgf or more was deemed to be ∘ (pass).-   Less than 126 kgf was deemed to be × (fail).

Abrasion Resistance Test: Shell Four-Ball Wear Test

The test equipment and test methods were such that a load of 40 kgf wasapplied in accordance with ASTM D4172, the oil temperature was 75° C.,the tester was rotated at 1200 rpm for 1 hour, and the diameter of anabrasion mark occurring at the point of contact was measured. WorkingExamples 1 and 12 and Comparative Examples 5 and 6 were subjected tothis test.

Evaluation of the test was carried out according to the followingcriteria:

-   An abrasion mark diameter of 0.50 mm or less was deemed to be ∘    (pass).-   An abrasion mark diameter of greater than 0.50 mm was deemed to be ×    (fail).

Storage Stability

The lubricating oil compositions of Working Examples 1-16 andComparative Examples 1-12 were allowed to stand for 1 day (24 hours) at25° C., after which the presence/absence of cloudiness or precipitationwas determined visually.

Examples in which cloudiness and precipitation had not occurred weredeemed to be ∘ (pass).

Examples in which cloudiness or precipitation had occurred are as shownin the tables.

With regard to storage stability, examples in which cloudiness orprecipitation had occurred were unsuitable as lubricating oilcompositions for sliding guide surfaces, and were therefore notsubjected to the other tests described above.

Test Results

The test results for the working examples and comparative examples areshown in Tables 1-3.

TABLE 1 Working Working Working Comparative Comparative ComparativeComparative Comparative Comparative Comparative Example Example ExampleExample Example Example Example Example Example Example 1 2 3 1 2 3 4 56 7 Base oil 1 98.75 98.75 99.0 99.0 99.0 99.75 99.88 97.0 Base oil 298.75 99.0 Base oil 3 Base oil 4 Base oil 5 Base oil 6 Base oil 7Additive 1-1 1.0 1.0 0.1 2.0 Additive 1-2 1.0 1.0 Additive 1-3 1.0 1.01.0 Additive 3 Additive 2-1 Additive 2-2 Additive 2-3 0.25 0.25 0.250.25 0.02 1.00 Additive 2-4 Additive 4 Additive 5 Additive 6 Coefficient0.093 0.091 0.089 0.114 0.113 0.116 0.135 0.092 0.107 of friction Flashpoint 270 260 258 264 264 268 246 268 266 (° C.) Four-ball 80 63 50 EP:ISL Four-ball 126 126 100 EP: WL Four-ball 0.41 0.73 0.81 wear Storage ∘∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ Cloudy stability

TABLE 2 Working Working Working Working Working Working WorkingComparative Comparative Comparative Example Example Example ExampleExample Example Example Example Example Example 4 5 6 7 8 9 10 8 9 10Base oil 1 98.82 98.3 98.77 98.7 98.7 98.7 98.65 97.45 99.95 Base oil 298.7 Base oil 3 Base oil 4 Base oil 5 Base oil 6 Base oil 7 Additive 1-11.0 1.0 1.0 1.0 1.0 1.0 1.0 Additive 1-2 1.0 1.0 Additive 1-3 Additive 3Additive 2-1 0.13 0.65 Additive 2-2 0.18 Additive 2-3 0.25 0.25 0.25Additive 2-4 0.25 Additive 4 0.3 1.5 Additive 5 0.05 0.05 0.05 0.05 0.050.05 0.05 0.05 0.05 0.05 Additive 6 Coefficient 0.100 0.105 0.097 0.0920.096 0.099 0.097 0.146 of friction Flash point 272 272 268 272 264 268246 272 (° C.) Four-ball EP: ISL Four-ball EP: WL Four-ball wear Storage∘ ∘ ∘ ∘ ∘ ∘ ∘ Precipitation Precipitation ∘ stability

TABLE 3 Working Working Working Working Working Working ComparativeComparative Example 11 Example 12 Example 13 Example 14 Example 15Example 16 Example 11 Example 12 Base oil 1 97.75 97.7 99 98.1 Base oil2 97.7 Base oil 3 97.7 Base oil 4 62 Base oil 5 35.7 Base oil 6 19.54Base oil 7 78.16 Additive 1-1 1.0 1.0 1.0 1.0 1.0 1.0 Additive 1-2Additive 1-3 Additive 3 0.6 Additive 2-1 Additive 2-2 Additive 2-3 0.250.25 0.25 0.25 0.25 0.25 Additive 2-4 Additive 4 Additive 5 0.05 0.050.05 0.05 0.05 0.05 Additive 6 1 1 1 1 1 1 1 1 Coefficient 0.092 0.0940.092 0.093 0.092 0.092 0.143 0.122 of friction Flash point 266 260 258242 226 224 250 214 (° C.) Four-ball 160 EP: ISL Four-ball 200 EP: WLFour-ball 0.40 wear Storage ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ stability

As shown in Table 1, the composition of Working Example 1, whichcontained base oil 1 and additives 1-1 and 2-3, had a low coefficient offriction of 0.093 and a high flash point of 270° C., and was found to beexcellent as a lubricating oil composition for a sliding guide surface.However, the composition of Comparative Example 1, which did not containadditive 2-3, passed in terms of flash point, but was found to beunsuitable due to having a high coefficient of friction of 0.114.

Similarly, comparing Working Example 2 and Comparative Example 2,Working Example 2 was suitable, whereas Comparative Example 2, whichdiffered from Working Example 2 by not containing additive 2-3, wasfound to be unsuitable. In addition, Working Example 3, which wasobtained by replacing base oil 1 in Working Example 2 with base oil 2,was good.

In addition, compositions which contained additive 1-2 but did notcontain additive 2, such as Comparative Examples 3 and 4, could notachieve good results. Comparative Example 5 differed from WorkingExample 1 by not containing additive 1-1, but passed in terms ofcoefficient of friction and flash point. However, Comparative Example 5exhibited the same WL as Working Example 1 of 126 kgf in the Shellfour-ball EP test, but had a lower ISL (63 kgf) than that of WorkingExample 1 (80 kgf) and had a worse abrasion mark diameter of 0.73 thanthat of Working Example 1 (0.41) in the Shell four-ball wear test, andwas found to be unsuitable.

Comparative Example 6 contained the same components as Working Example1, but contained lower quantities of additive 1 and additive 2, andfailed in terms of coefficient of friction, ISL and WL in the Shellfour-ball EP test and abrasion mark diameter in the Shell four-ball weartest, and was significantly inferior to Working Example 1.

Comparative Example 7 contained the same components as Working Example1, but contained higher quantities of additive 1 and additive 2 andexhibited cloudiness in the storage stability test, and was unsuitable.Because cloudiness occurred, as mentioned above, the other tests werenot carried out.

As shown in Table 2, the compositions of Working Examples 4-8 allcontained base oil 1 and additive 1-1, all contained additive 5, andvaried in terms of the type and content of additive 2, but all passed interms of coefficient of friction and flash point, and were found to besuitable.

Working Example 9 differed from Working Example 7 by containing additive1-2 instead of additive 1-1, and Working Example 10 differed fromWorking Example 9 by containing base oil 2 instead of base oil 1, butWorking Examples 9 and 10 passed in terms of coefficient of friction andflash point, and were found to be suitable.

Comparative Example 8 differed from Working Examples 4-8 by containing0.3 mass % of additive 4 (behenic acid) instead of additive 2, andComparative Example 9 differed from Working Examples 4-8 by containing1.5 mass % of additive 4 (behenic acid) instead of additive 2, butComparative Examples 8 and 9 underwent precipitation in the storagestability test, and were therefore undesirable. In addition, becauseprecipitation occurred, the other tests were not carried out.

Comparative Example 10 contained additive 5 in base oil 1, but did notcontain additive 1 or additive 2, and passed in terms of flash point,but was found to be unsuitable due to exhibiting an extremely highcoefficient of friction of 0.146.

As shown in Table 3, Working Example 11 was obtained by adding additive6 to the composition of Working Example 1, and passed in terms ofcoefficient of friction and flash point.

Working Example 12 was obtained by adding additive 5 and additive 6 tothe composition of Working Example 1, and passed in terms of coefficientof friction and flash point, and exhibited a similar abrasion markdiameter to Working Example 1 in a Shell four-ball wear test, butexhibited higher values for ISL and WL in a Shell four-ball EP test, andwas therefore found to be more preferable in cases where high extremepressure properties are required.

Working Examples 13-16 were obtained by replacing base oil 1 used inWorking Example 12 with other base oils, and all passed in terms ofcoefficient of friction, and Working Examples 15 and 16 were slightlyinferior in terms of flash point, but still passed.

Comparative Example 11 differed from Working Example 11 by notcontaining additive 1 or additive 2, and passed in terms of flash point,but was unsuitable due to exhibiting a high coefficient of friction,like Comparative Example 10.

Comparative Example 12 differed from Working Example 12 by containingadditive 3, which is a phosphonic acid ester having a benzene ring,instead of additive 1-1, and failed in terms of coefficient of frictionand flash point, and was found to be unsuitable.

1. A lubricating oil composition for a sliding guide surface comprising:a base oil that is a base oil of group Ito group IV in the API base oilcategories or a mixture thereof, a phosphonate represented by formula 1

wherein R₁ denotes a saturated or unsaturated alkyl group having 12-22carbon atoms, and R₂ denotes a saturated or unsaturated alkyl grouphaving 1-18 carbon atoms, and a fatty acid represented by formula 2R₃COOH   (2) wherein R3 denotes a saturated or unsaturated alkyl grouphaving 7-17 carbon atoms.
 2. A lubricating oil composition for a slidingguide surface according to claim 1, wherein the phosphonate is presentin a quantity of not less than 0.2 mass % but less than 2 mass %relative to the overall quantity of the lubricating oil composition, andthe fatty acid is is present in a quantity of not less than 0.03 mass %but less than 1 mass % relative to the overall quantity of thelubricating oil composition.
 3. A lubricating oil composition for asliding guide surface according to claim 1, wherein the base oil is abase oil belonging to group III and/or a base oil belonging to group IVin the API base oil categories.
 4. A lubricating oil composition for asliding guide surface according to claim 1, which further comprises ananti-wear agent and/or a metal deactivator.
 5. (canceled)
 6. Alubricating oil composition for a sliding guide surface according toclaim 1, wherein the fatty acid is present in a quantity of not lessthan 0.1 mass % and not more than 0.7 mass % relative to the overallquantity of the lubricating oil composition.
 7. A lubricating oilcomposition for a sliding guide surface according to claim 1, whereinthe phosphonate comprises one or more phosphonates selected from thegroup consisting of dimethyldodecyl phosphonate, dimethyltridecylphosphonate, dimethyltetradecyl phosphonate, dimethylpentadecylphosphonate, dimethylhexadecyl phosphonate, dimethylheptadecylphosphonate, dimethyloctadecyl phosphonate, dimethylnonadecylphosphonate, dimethyleicosyl phosphonate, tridodecyl phosphonate,tritridecyl phosphonate, tritetradecyl phosphonate, tripentadecylphosphonate, trihexadecyl phosphonate, triheptadecyl phosphonate,trioctadecyl phosphonate (tristearyl phosphite: tautomer) and trioleylphosphonate.
 8. A lubricating oil composition for a sliding guidesurface according to claim 1, wherein the fatty acid comprises one ormore more fatty acids selected from the group consisting of caprylicacid, capric acid, lauric acid, myristic acid, palmitic acid, stearicacid, isostearic acid, oleic acid, linoleic acid and linolenic acid. 9.A method comprising: applying a lubricating oil composition to a slidingguide surface of a machine tool, wherein the lubricating oil compositioncomprises: a base oil that is a base oil of group Ito group IV in theAPI base oil categories or a mixture thereof, a phosphonate representedby formula 1

wherein R₁ denotes a saturated or unsaturated alkyl group having 12-22carbon atoms, and R2 denotes a saturated or unsaturated alkyl grouphaving 1-18 carbon atoms, and a fatty acid represented by formula 2R₃COOH   (2) wherein R3 denotes a saturated or unsaturated alkyl grouphaving 7-17 carbon atoms.
 10. A method according to claim 9, wherein thephosphonate in the lubricating oil composition is present in a quantityof not less than 0.2 mass % but less than 2 mass % relative to theoverall quantity of the lubricating oil composition, and the fatty acidis is present in a quantity of not less than 0.03 mass % but less than 1mass % relative to the overall quantity of the lubricating oilcomposition.
 11. A method according to claim 9, wherein the base oil isa base oil belonging to group III and/or a base oil belonging to groupIV in the API base oil categories.
 12. A method according to claim 9,wherein the lubricating oil composition further comprises an anti-wearagent and/or a metal deactivator.
 13. A method according to claim 9,wherein the fatty acid in the lubricating oil composition is present ina quantity of not less than 0.1 mass % and not more than 0.7 mass %relative to the overall quantity of the lubricating oil composition. 14.A method according to claim 9, wherein the phosphonate in thelubricating oil composition comprises one or more phosphonates selectedfrom the group consisting of dimethyldodecyl phosphonate,dimethyltridecyl phosphonate, dimethyltetradecyl phosphonate,dimethylpentadecyl phosphonate, dimethylhexadecyl phosphonate,dimethylheptadecyl phosphonate, dimethyloctadecyl phosphonate,dimethylnonadecyl phosphonate, dimethyleicosyl phosphonate, tridodecylphosphonate, tritridecyl phosphonate, tritetradecyl phosphonate,tripentadecyl phosphonate, trihexadecyl phosphonate, triheptadecylphosphonate, trioctadecyl phosphonate (tristearyl phosphite: tautomer)and trioleyl phosphonate.
 15. A method according to claim 9, wherein thefatty acid in the lubricating oil composition comprises one or more morefatty acids selected from the group consisting of caprylic acid, capricacid, lauric acid, myristic acid, palmitic acid, stearic acid,isostearic acid, oleic acid, linoleic acid and linolenic acid.